MIDI has RPN (registered parameter numbers) dedicated to coarse and fine pitch tuning (https://www.midi.org/specifications-old/item/table-3-control-change-messages-data-bytes-2). This would need a specialised editor in VMPK, but it is doable. For this mechanism to work, the synthesizer must also be able to receive and understand these parameters. There are synthesizers that use system exclusive messages to adjust the tuning (there are universal sysex messages: https://www.midi.org/specifications-old/item/table-4-universal-system-exclusive-messages)